1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a zoom lens system in which the fluctuation of aberration caused by the movement of the zooming lens component accompanying the zooming operation is independently compensated.
2. Description of the Prior Art
In a zoom lens system, it is required to sufficiently compensate or correct the aberration over the whole range of zooming. In order to correct the aberration, it has generally been proposed to divide the lens system into two groups, one of which compensates the fluctuation of the aberration caused by the zooming operation and the other of which corrects the aberration on the focussing plane which determines the quality of the focussed image. The lens system in which the fluctuation of aberration caused by the zooming operation is to be compensated is divided into a focussing lens component, a variator lens component, and a compensator lens component. It is, therefore, necessary that the lens system should be so formed that the fluctuation of aberrations including spherical aberration, coma, astigmatism, distortion, longitudinal chromatic aberration and lateral chromatic aberration may all be compensated by the three lens components. However, it is very difficult or actually impossible to completely correct these aberrations by use of the three lens components.
Taking the focussing lens component for instance, the amount of the spherical aberration produced thereby varies at a ratio three times as large as the ratio of zooming, assuming that the brightness does not vary between the extremities of the range of zooming. The position of the entrance pupil changes along with the zooming operation and is generally separated from the focussing lens component since the stop is positioned behind the zooming lens component. Accordingly, the amount of the distortion, astigmatism and lateral chromatic aberration becomes large and complicatedly varies over the range of zooming. Under such circumstances, the aberrations produced by the focussing lens component are mainly corrected by the variator lens component. However, on the other hand, the aberrations produced by the variator lens component also vary complicatedly over the range of zooming because the variator lens component also moves along the optical axis for varying the focal length.
Therefore, it is impossible to completely correct the fluctuation of all the aberrations over the whole range of the zooming operation. Accordingly, in the actual zoom lens system, various aberrations are generally reduced to be smaller than an allowable predetermined small range to obtain an image of as high quality as possible. The range which must be accepted as allowable becomes inevitably larger as greater demands are put on the zoom lens regarding various performance factors. In the actually manufactured zoom lenses, therefore, it often happens that the finally obtained image, in which the various aberrations are limited to their minimum by use of the highest level of optical design engineering available, still suffers from the remaining aberration which exceeds the practically allowable limit.
The fluctuation of the aberration is considerably complicated, since a number of kinds of aberrations fluctuate in the course of the zooming operation and the diameter of the off-axis luminous flux changes together with the zooming and focussing operations, and further the fluctuation of aberration caused in the focussing lens component by the variation in the distance of the subject varies together with the zooming operation.